Calculating chain length for SRAM 1x

The way I've always calculated chain length for Shimano 2x systems is either:

1. Use an online calculator like http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

2. Wrap a chain around the biggest chainring and biggest sprocket to see where they meet. Add 2 pins, and cut the chain there.

I recently bought a bike that uses SRAM Rival 1x gears, and would like to know how to calculate the right chain length for it, primarily so that I can make sure I avoid buying a replacement chain that is too short out of the packet.

The SRAM manual (page 14 of https://www.sram.com/globalassets/document-hierarchy/user-manuals/sram-road/drivetrain/95-7518-007-100-1x-rev-a-road-system-user-manual-eeu.pdf) suggests wrapping the chain around the biggest sprocket and the chainring, and then adding 4 pins. So that's similar to what I've done in the past for Shimano.

Using that method, it looks like I should have use a 55 inch chain. The online chain length calculator also agrees with 55 inch.

However, the current chain on there (came like that from the factory) has an extra 6 pins of length on top of what either of my normal methods think it should be. It is 58 inches.

The bike has ridden fine over the past year, with no noticeable issues. When I come to replace the chain should I replace like for like and go with 58 inches? Or should I trust the SRAM manual and online calculator and go with 55 inches? I'm loathe to "just try 55 inches", as once I've cut the chain it'll be a bit of a pain if I decide it's actually too short and I should go back up to 58.

The SRAM manual suggests wrapping the chain around the biggest sprocket and the chainring, and then adding 4 pins

This. Big to big, plus 2 whole links. There's flexibility both ways so it may have been fine with a few extra links do you can have a slack chain in the smallest cogs, the rest would be fine

There's no correct answer. Usually the instructions you find are for finding the shortest chain length that will work, but generally the system works just as well with a chain that's a few links longer than that, as long as it's not too long.

Shift to the smallest cog and see how well the cage is taking up the slack. If the cage is all the way back or the upper jockey wheel is touching the lower run of chain (or close to touching), your chain is too long.

Small/small; a little tension on the derailleur. Done.
What's with all the adding and subtracting?

THIS is the right way, for any system. And if in doubt, cut longer. You can always remove a link when everything it's set up and working.

Isn't there then a risk that the chain is too short for the large/large combination?

Consequences if a chain that is too short seem much worse than one that is too long, based purely on internet horror stories.

Nope. Unless you ham handed and pull too much tension on the cage. If anything, it gives you breathing room.

I've used this daily from bling bikes, to 1x to Bromptons to everything in between.

Yeah but if you’ve fitted too large a cassette or too short a cage and you set chain length based on the smallest combination, as hazzelfrazzel points out, you’re going to run into trouble when your chains too short and you grind to a halt with your jockey wheels in your spokes. In practise it would rarely happen but I feel like it’s just safer to suggest setting chain length based on the biggest combination

fitted too large a cassette or too short a cage

But then that's your problem, not the chain length.

What's with all the adding and subtracting?

That’s not much consolation when you’re lying in the road.
If it’s less nasty for your chain to be too long than too short, why not suggest to people asking, such as in this thread, to start with the biggest combination and go from there rather than the way of a qualified mechanic who’ll spot a problem like the wrong cassette before the chain comes out of the box.

Surely people test their gears when changing drive train parts, before going on a ride? They should, anyway.

I'm not saying I recommend the small-small method per se. But if you've done everything right, it's a perfectly fine method. I do agree the big-big method is safer.

If you've done so, it doesn't matter if you've done chain calculations or small-small, you're still fucked no?
You're either working to the recommended manufacturer recommendations or you're pushing limits and thus pushing working parameters. At that point, you really shouldn't be online asking for recommended methods, as you're far outside the scope of recommended calculations.

Doesn't matter what you're doing, always test before use. Then shakedown, test again, then ride.

I've also read what OP was saying, he's on a 1x system, so my recommendation isn't misplaced.

The Lennard Zinn book specifically says for the SRAM 1x setups to go big-big with 2x full links overlap with inner at both ends, then add your quicklink for a total of 2.5 links of overlap.

A quick link is a whole link. Whenever people say 'two full links', they're actually saying four links. Maybe the misuse of the word link is causing people to cut their chains too short...

The Lennard Zinn book...

A book is already out of date by the time you pick it up.

I've been really impressed by this book to be honest.

With respect to a definition of a link indeed it is confusing and used differently by different people. Zinn does have a drawing of what he means by 'a whole link' so at least there is no confusion for the reader.

It says it on the packaging. I never heard anyone complain they only got 50% of the advertised amount of chain.